Thermoset resins, including unsaturated polyesters, are commonly employed in a variety of fabrications, such as casting materials, fiber reinforced materials and coatings. Unsaturated polyester resins are the condensation products of dicarboxylic acids or anhydrides with difunctional alcohols, typically dissolved in a reactive diluent such as styrene. Since the commercial inception of unsaturated polyester resins, styrene has been a primary reactive diluent due to its low cost, availability, ease of use and resulting mechanical and physical properties. However, in recent years, government regulations have restricted styrene emissions in open molding facilities, citing harmful environmental and health effects associated with styrene.
A number of alternatives has been sought to reduce emissions and as substitutes for styrene. The addition of paraffin to unsaturated polyester resins has been shown to reduce emissions by forming a thin film on the resin surface. While suitable for some applications, this solution is unacceptable in others due to a decreased interlaminate adhesion. In most cases, low volatile acrylate and methacrylate monomers have been inadequate as replacements for styrene in unsaturated polyester resins since atmospheric oxygen severely retards polymerization, resulting in an under-cured “tacky” surface. Furthermore, the poor copolymerization of most acrylates and methacrylates with maleate and fumarate moieties in the polyester backbone often results in inferior mechanical properties compared to styrenated unsaturated polyester resins.
Previous patents have described non-styrenated unsaturated polyester resin compositions. For example, U.S. Pat. No. 5,747,597 describes a curable resin comprised of an unsaturated polyester, an epoxymethacrylate and/or an urethane methacrylate, combined with oligoalkyl ether-monoalkoxy methacrylates. The resin is described in many cases to air dry (i.e., “dry-to-touch”) after the addition of wax, free radical initiator, catalyst and exposure to elevated temperatures (130-145° F.). Tensile testing of the cast resin reveals high elongation and low strength properties, which is in sharp contrast to data measured using the same base polymers dissolved in styrene that demonstrated considerably higher tensile strengths.
European Patent Publication No. WO 99/23122 demonstrates the use of a hydroxyethyl methyl methacrylate, hydroxyethyl propyl methacrylate and/or urethane hydroxyethyl methacrylate with polyesters and vinyl esters. The resulting tensile and flexural properties of the cured products are comparable to styrenated unsaturated resins, but the tack-free cure in thin films and thin film laminates tends to be a problem with these resins. Additionally, the storage stability of these resins tends to be poor.
Other related U.S. patents are U.S. Pat. Nos. 5,908,875; 5,739,240; 6,277,939; 5,773,531 and 6,037,385.
An ongoing goal in the unsaturated polyester industry has been to develop improved non-styrenated, free radical curable laminating and coating resins. Desired properties include processing similar to current styrenated resins, benefits in mechanical and physical properties and tack-free cure in thin film laminates and coatings.
Unsaturated polyester coatings known as gel coats present a problem with high emissions since they are usually applied by spray-up, and the surface area to volume ratio of the coating on the composite part is very high. Some publications have described low styrene or low hazardous air pollutant unsaturated polyester coating resins, but very few publications have described low volatility gel coats which are free of components that are analogues of styrene. Compounds related to styrene (such as alpha-methyl styrene, vinyl toluene, divinyl benzene and the like) similarly exhibit photochemical reactivity and volatility. These negative features make these compounds undesirable for use in a coating resin. There are major obstacles in successfully preparing and utilizing low vapor pressure polyester gel coats lacking styrene or analogs of styrene.